In general, sandwich panels are constituted by high-density high-strength upper/lower face sheets and low-density low-strength cores. When compared to general panels formed of a uniform one material, sandwich panels have been widely used in structures of which lightweight and high strength and stiffness are required because of high strength and stiffness to weight ratio. Thus, sandwich panels are being widely used in furniture, civil engineering, and constructive materials as well as high-priced advanced structures such as wings for airplane and the bottom of passenger room. A fiber reinforced plastic (FRP) face sheet and a honeycomb core may be considered as the most ideal combination in sandwich panels. However, since the honeycomb core has a closed inner space, the inner space of the honeycomb is not used. Also, since the core and the face sheet are connected to each other by using an adhesive having low adhesion strength, the sandwich panels may be vulnerable to, particularly, a fatigue load.
In recent, a lightweight structure having a periodical truss structure has been developed as a material for cores. Since the lightweight structure has a truss structure designed to provide optimum strength and stiffness through accurate mathematical/mechanical calculation, the lightweight structure may have superior mechanical properties. Here, the most general truss in structural shape may be a pyramid truss and an octet truss (see R. Buckminster Fuller, 1961, U.S. Pat. No. 2,986,241). Recently, as a modification of the octet truss, Kagome truss has been known (see S. Hyun, A. M. Karlsson, S. Torquato, A. G. Evans, 2003. Int. J. of Solids and Structures, Vol. 40, pp. 6989-6998). In this case, if the whole members constituting the truss have the same length when thin and long members having the same section constitute the truss, each of truss elements constituting the Kagome truss may have just a length equal to half of that of each of truss elements constituting the octet truss. Thus, buckling that is a main fracture phenomenon of trusses may be more effectively restricted. Also, even though the buckling occurs, the truss may more stably collapse. For example, each of the pyramid, octet, and Kagome trusses is three-dimensionally illustrated in FIG. 1. Since a truss has an opened inner space, the truss may be used as various purposes such as a fluid storage or path and a thermal transfer medium. In addition, if the truss is used as a sandwich core, the truss may obtain strength to weight ratio that comes close to that of the sandwich panel (see A. G. Evans, J. W. Hutchinson, N. A. Fleck, M. F. Ashby, H. N. G. Wadley, 2001, Progress in Materials Science, Volume 46, Issues 3-4, pp. 309-327). For this reason, the truss is coming into the spotlight.
Methods for manufacturing a truss-type porous lightweight structure are known as follows. First, there is a method in which a truss structure is manufactured using a resin, and then a metal is molded by using the truss structure as a mold (see S. Chiras, D. R. Mumm, N. Wicks, A. G. Evans, J. W. Hutchinson, K. Dharmasena, H. N. G. Wadley, S. Fichter, 2002, International Journal of Solids and Structures, Vol. 39, pp. 4093-4115) (hereinafter, referred to as “Prior art 1”). Second, there is a method in which a thin metal plate is punched at a predetermined interval to manufacture a mesh-shaped plate, the mesh-shaped plate is bent to form a truss intermediate layer, a face plate is attached on each of upper and lower portions of the truss intermediate layer (see D. J. Sypeck and H. N. G. Wadley, 2002, Advanced Engineering Materials, Vol. 4, pp. 759-764) (hereinafter, referred to as “Prior art 2”). In this case, when it is intended to manufacture a multilayered structure including at least two layers, the truss intermediate layer that is bent as described above is attached on an upper face plate, and then a face plate is attached again on the truss intermediate layer. Third, there is a method in which wires disposed in two directions perpendicular to each other are woven in a mesh shape to form iron meshes, and then the iron meshes are stacked on each other (see D. J. Sypeck and H. G. N. Wadley, 2001, J. Mater. Res., Vol. 16, pp. 890-897) (hereinafter, referred to as “Prior art 3”).
However, Prior art 1 may have a complicated manufacturing process and an expensive manufacturing cost. Also, since Prior art 1 is applied to only a metal having superior moldability, an application scope may be narrow. In addition, the resultant product may have many defects on the cast structure and low strength. According to Prior art 2, a large amount of materials is lost when the thin metal plate is punched. Even though there is no special problem when the truss intermediate layer is manufactured as one body, if a plurality of truss intermediate layers are stacked, the number of bonded portions may increase, and thus there are disadvantageous in aspects of bonding costs and structural strength. Also, in the case of Prior art 3, since the truss does not have an ideal structure such as the tetrahedron or pyramid structure, the truss may have inferior mechanical strength. In addition, since the iron meshes should be stacked and bonded in the same method as that of Prior art 2, the number of bonded portions may increase, and thus there are disadvantageous in aspects of bonding costs and structural strength.
FIG. 2 is a view of a structure manufactured using Prior art 3. More particularly, even though it is known that manufacturing costs can be reduced in Prior art 3, as shown in FIG. 3, since wires disposed in the two directions perpendicular to each other are combined with each other as if fibers are woven, the truss may not have the optimally ideal structure in mechanical or electrical properties, such as the three-dimensional (3D) octet truss and the 3D Kagome truss. Also, since the number of bonded portions increases, there are disadvantageous in aspects of bonding costs and structural strength.
Thus, to solve the above-described problems of Prior arts 1, 2, and 3, a 3D porous lightweight structure in which a manufacturing method in which a wire group in which six-directional continuous wires having an azimuth of about 60 degrees to about 120 degrees are crossed with each other in a space to form a truss having a shape similar to that of the ideal Kagome or octet truss, and a manufacturing method thereof are developed by the present inventors, i.e., Kang Ki-ju et al. The structure and manufacturing method are concretely disclosed in KR Patent Registration No. 0708483. Also, a 3D porous lightweight structure woven using a spiral wire, in which a continuous wire is formed in a spiral shape, and then, the spiral-shaped wire is rotated and inserted to manufacture the 3D porous lightweight structure, and a manufacturing method thereof are proposed as a method for more effectively manufacturing the 3D porous lightweight structure by the same inventors. The structure and manufacturing method are concretely disclosed in KR Patent Publication No. 2006-0130539.
FIG. 3 is a view of a structure having a shape similar to that of the 3D Kagome truss of FIG. 1 and assembled using a spiral wire. The 3D multilayered truss structure of FIG. 3 having the shape similar to that of the Kagome truss and manufactured by using the spiral wire may have several advantages such as superior mechanical properties and mass productivity due to the successive process when compared to the conventional truss structure.
A method for manufacturing a new 3D porous lightweight structure which may be manufactured using a spiral wire and have a shape different from that of the Kagome truss is proposed in KR Patent Application No. 10-2009-0080085 by the same inventors. FIG. 4 is a view illustrating an example of the truss structure assembled using the spiral wire, which is disclosed in KR Patent Application No. 10-2009-0080085.
Also, a new 3D lattice truss structure which may be manufactured using the spiral wire and have a structure in which only two wires meet each other at a wire cross point to manufacture the 3D lattice truss structure by using a spiral wire having a more less spiral radius, and a manufacturing method thereof are proposed in KR Patent Application No. 10-2010-00 59690 by the same inventors. FIG. 5 is a view illustrating an example of the truss structure assembled using the spiral wire, which is disclosed in KR Patent Application No. 10-2010-00 59690.
A similar truss structure using a continuous wire is being evaluated as a metal sandwich panel having superior mechanical performance in strength to weight ratio and high mass productivity (see Yong-Hyun Lee, Byeong-Kon Lee, Insu Jeon and Ki-Ju Kang, 2007, Acta Materialia, Vol. 55, pp. 6084-6094. Yong-Hyun Lee, Ji-Eun Choi and Ki-Ju Kang, 2009, Materials and Design, Vol. 30, Issue 10, pp. 4459-4468). Since wire crossing portions and portions contacting face sheets of the similar truss structure formed of the metal are bonded using brazing or welding, the truss structure may have bonding strength as superior as that of a mother material of the wire or face sheet. However, in a case where a similar truss structure using a wire such as FRP or tungsten in which the performing of the welding or brazing is difficult is manufactured, only a method using a synthetic resin adhesive may be utilized to bond wire crossing portions and portions contacting face sheets to each other. In this case, bonding strength may be significantly inferior to that of the welded or brazed metal. Particularly, the portions bonded to the face sheets may be vulnerable to separate cores from the face sheets.
A process for manufacturing a sandwich panel from an intermediate product of an existing velvet weaving process has been developed by Belgium's and Germany's research teams (see Drechsler K, Brandt J, Arendts F J. Integrally woven sandwich structures. Proc ECCM-3, Bordeaux 1989. p. 365-371. Verpoest I, Bonte Y, Wevers M, de Meester P., Declercq P. 2.5D- and 3D-fabrics for delamination resistant composite structures. Proc European SAMPE, Milano, Italy 1988. p. 13-21). FIG. 6A is a schematic view of an existing velvet weaving process. A 3D fabric having a sandwich shape as shown at a middle portion of FIG. 6A is manufactured through a manner in which a weft is reciprocated between top and bottom warps as shown at a left side of FIG. 6A. Then, as shown at a right side of FIG. 6A, the weft between the top and bottom warps is bisected by using a sharp knife to manufacture a velvet cloth having soft hairs on one surface thereof. A process in which a 3D fabric that is an intermediate product is manufactured using a fiber for reinforcing a composite such as a glass fiber is manufactured, and then, a syntactic resin such as epoxy is injected into the 3D fabric to manufacture a sandwich panel has been developed by the two research teams. FIG. 6B is a photograph of the sandwich panel manufactured through the above-described method. The sandwich panel may be called a “woven sandwich-fabric panel”, “integrally woven sandwich”, or “woven textile sandwich”. Since a core and a face sheet are bonded by using the weft woven with the warps constituting the face sheet and continuously connected to each other, but using the syntactic resin adhesive, the sandwich panel may have resistance with respect to the separation between the core and the face sheet may be significantly superior when compared to that of the existing composite/honeycomb sandwich panel. However, since the weft constituting the core is bent and does not have a truss shape, the sandwich panel may have low strength and stiffness with respect to compression and shearing. Thus, the sandwich panel is being utilized in use for which a high load is not applied, such as a use for a partition of a building or furniture (see van Vuure A W, Ivens J A, Verpoest I. Mechanical properties of composite panels based on woven sandwich-fabric preforms. Composites Part A 2000; 31: 671-680).